Sewage Sludge: An Important Biological Resource for Sustainable Agriculture

Pakistan ranks among one of those countries which almost depends on their agriculture. The 70% population of Pakistan is either directly or indirectly interconnected to the agriculture sector. It contributes 21.4% in the GDP of the Pakistan and according to survey 45% of the rural population is directly depends on Agriculture to make their basic needs of life. But now a day a trend of using sewage sludge is established to enhance the productive abilities of soil. Intensive farming generally requires huge addition of organic matter to maintain fertility and enhance crop yields. Sewage sludge/biosolids are by-products of municipal and industrial wastewater treatment and a rich source of organic nutrients. Sewage sludge contains high content of organic matter, macro- and micro-nutrients, can be used as fertilizer/soil conditioner for food, vegetable crop, horticultural plants and pasture, which in most cases can be beneficially recycled. In the past sewage sludge was regarded as a waste product due to the presence of high level of contaminants such as pathogens, pollutants and synthetic materials discharged in sewage lines from homes and industries, which were often incinerated, dumped in land fill. As a result of rapidly increase in population, urbanization and industrialization, wastewater production and sewage sludge generation have increased manifold. Due to high cost of mineral fertilizers and tremendous increase in synthetic fertilizer’s prices, there is an increasing trend of using sewage sludge in agriculture, especially under intensive cropping systen in arid and semi-arid regions of the country. Therefore, application of sewage sludge to agricultural soils may be sustainable and economical due to nutrient cycling and disposal of sewage sludge. However, there may be a risk in use of sewage sludge due to potentially harmful contents present in the sludge such as heavy metals and pathogens.

Sewage sludge consists of by-products of wastewater treatment. It contains water, inorganic and organic materials removed from wastewater coming from various sources, storm water runoff from roads and other paved area, through physical, biological, and/or chemical treatments. Sewage sludge is also referred to as bio solids. Various agencies have put forth different definitions of Bio solids. Bio solids are nutrients rich organic materials resulting from the treatment of domestic sewage in a treatment facility. Some regards bio solids as stabilized organic solids derived from biological wastewater treatment process, which can be managed safely to utilize their nutrients on sustainable basis, and used for soil conditioning, energy, or other value. The use of bio solids/sewage sludge in agriculture is gaining popularity as a source of waste disposal. Bio solids/sewage sludge generally contains useful compounds of high potential environmental value.

They also contain useful concentration of organic matter, nitrogen, phosphorus and potassium and to lesser extent, calcium, sulphur and magnesium. The availability of phosphorus content in the year of application to agricultural lands is above 50% and is independent of prior sludge treatment. Nitrogen availability is more dependent on sludge treatment. Untreated liquid sludge and dewatered treated sludge release nitrogen slowly with some benefits to crop being realized over a relatively long period. Anaerobically digested sludge has high contents of ammonia, which is readily available to plants. Thus it is a nutrient enriched fertilizer. The organic matter content in sludge can improve soil physical, chemical, and biological properties with ensuring better cultivation and aqui-ferrous capacity of soil, especially when applied in the form of dewatered sludge cake. Bio solids reduce runoff and increase surface retention of rain water .Organic nitrogen in sludge is much less likely to cause ground water pollution than chemical nitrogen fertilizer. The application of sewage sludge to agricultural land is the best way of recycling the nutrients present in it. Therefore, sewage sludge may be considered an important biological resource for sustainable agriculture. It produces favorable plant yield response, when used as an organic fertilizer. Research on land application of sewage sludge has mainly focused on its role in introduction of heavy metals in the food chain. However, it has been shown that sewage sludge application at recommended rates increased microbial activity in soil and tied up the heavy metals making them unavailable to plant and soil.